The present invention relates generally to a locking assembly used in conjunction with nuclear reactors to normally lock in place an upper support plate disposed at the top of a fuel bundle, and to selectively permit the easy removal of such support plate when it is necessary or desirable to remove one or more of the fuel rods in the fuel bundle.
There are several types of nuclear reactors used to generate power, one of which is the pressurized water reactor (PWR). In a PWR the reactor core contains an array of fuel bundles or assemblies comprised of fuel rods containing uranium.
PWR units in the United States generally operate for approximately 12 to 18 months after which the plant is routinely shut down for refueling so that all systems and components can be checked to ensure safety and reliability. Occasionally, fuel will fail during the 12 to 18 months of power operation. Fuel is considered failed when a fuel rod wall is breached and radioactive isotopes are released into the water which cools the fuel during power operation. Failure can result in a number of ways, such as debris in the cooling water fretting the fuel rod in a localized area, or flaws introduced during fuel fabrication.
In the past, a small percentage of failed fuel was acceptable, but, recently, emphasis has been placed on eliminating the continued operation of any known failed fuel. When fuel fails, radioactive isotopes are released into the coolant water and should the level of these radioactive isotopes indicate that there is failed fuel, such failed fuel should be promptly removed.
Rather than remove an entire fuel assembly from further operation, a more economical solution is to replace only the failed fuel rods with structurally sound replacement rods and return the assembly back to operation. The fuel rods are replaced by disconnecting an upper support plate or upper end fitting from guide tubes forming part of the fuel assembly, thereby allowing the fuel rods to be removed. The replacement or removal of failed fuel rods occurs during the outage when time to perform such work is limited, and, therefore, a need exists for fuel assembly features which allow replacement to be performed as quickly as possible. Early fuel assembly designs did not provide a means for replacement since some failed fuel was acceptable. However, as emphasis was placed on removal of failed fuel, designs were developed to allow replacement.
For example, Long et al U.S. Pat. No. 4,064,004 discloses an assembly mechanism which permits the upper support plate to be removed from guide tubes and which includes a pair of cooperating movable members, one of which is operable to be moved to one position at which the upper support plate is normally maintained in place on the guide tubes by lugs, and to be selectively rotated to a second position at which a slot formed in the support plate can be aligned with such lugs to permit removal of the support plate. The other movable member is axially movable and is urged by a coil spring to a normal position at which it prevents the rotatable member from being moved to its aligned position with respect to the support plate slot. In this system, the cooperating locking members are locked together at the support plate release position but they are not locked to the guide tube after the support plate is removed. Therefore, there is no assurance that the locked members will remain at the same orientation relative to the guide tube after the support plate is removed, and they are free to move to a different position on the guide tube which will require that all of the locked members be carefully aligned with the slots on the support plate when the support plate is repositioned on the ends of the guide tubes.
In accordance with the present invention, a simplified locking assembly is provided which overcomes the aforesaid drawback of known locking mechanisms, and which provides an arrangement that is easy to operate while providing a secure lock for the upper support plate.